The study, published in the journal Nature, found that the giant, 900-gene virus Acanthamoeba polyphaga was infected by a smaller, 21-gene virus named Sputnik. The size of the Acanthamoeba virus itself is remarkable, as the virus is three times larger than largest previously discovered virus and larger than even some bacteria.
Acanthamoeba is a bacteriophage, a class of virus that infects bacteria. In this case, the bacterium is the amoeba. Acanthamoeba embeds itself in the amoeba and takes over the reproductive mechanism of the cell to make more viruses. Sputnik, however, is the first in a new class of viruses named virophages by the authors of the paper. Sputnik embeds itself in the DNA of Acanthamoeba, taking advantage of Acanthamoeba in the same way that virus takes advantage of its amoeba host. Once infected, an Acanthamoeba produces fewer new viruses. In effect, the virus gets sick.
Another similarity comes from a genetic analysis of Sputnik that showed some of Sputnik’s genes actually came from other viruses. That discovery implies that virophages may transfer genes between the viruses they infect similar to the way bacteriophages transfer genes between bacteria. In bacteria, the gene transfer facilitated by bacteriophages is an important catalyst for evolution.
The discovery of a virus that infects other viruses also raised the debate about whether or not a virus is alive. For many scientists, viruses do not qualify as alive due to their inability to reproduce without the help of a host cell. However, with the discovery of Sputnik, Jean-Michel Claverie, a virologist at the lab where Sputnik was discovered, suggest that if something can become sick, it must be alive.
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